1. Field of the Invention
The present invention relates to a capacitor microphone and an impedance converter therefor. More specifically, the present invention relates to a capacitor microphone and an impedance converter therefor using a vacuum tube as an impedance converting element improved for stable operation and avoidance of sound quality degradation.
2. Description of the Related Art
Capacitor microphones have small effective capacitance and high output impedance. Thus, for an output signal from a capacitor microphone, high input impedance is required to assure frequency response at a low frequency domain, as well as at a high frequency domain or a mid-frequency domain. Upon feeding an output signal from a capacitor microphone to an amplifier through a cable and the like, the output impedance of the capacitor microphone needs to be lowered. Therefore, capacitor microphones incorporate an impedance converter having high input impedance and low output impedance. A field-effect transistor (FET) is widely used as an impedance conversion element incorporated in a capacitor microphone.
A capacitor microphone is known that uses a vacuum tube as an impedance conversion element for obtaining higher sound quality and maximum output level (see, for example U.S. Pat. No. 6,453,048). U.S. Pat. No. 6,453,048 discloses, as an embodiment of the invention, an impedance converter including: a grounded plate amplifier tube; and a bias circuit that generates a bias voltage to be applied to the grid of the amplifier tube. The bias circuit includes: a first diode that applies a bias voltage to the grid of the amplifier tube so that a current flows to the grid; a second diode connected in inverse and parallel with the first diode; and a third diode provided between the cathode of the amplifier tube and a load resistance so that a current flows from the cathode of the amplifier tube to the load resistance. With a plate current flowing in the amplifier tube, a voltage generated in the third diode is applied to the grid of the amplifier tube as a bias voltage via the first or the second diodes.
By feeding a sound signal as a result of conversion by a capacitor microphone unit to the grid of the amplifier tube, an output signal from the capacitor microphone having high input impedance can be output as a low output impedance sound signal.
The impedance converter disclosed in U.S. Pat. No. 6,453,048 outputs a signal with a triode vacuum tube in cathode follower connection. A cathode follower realizes high input impedance and low output impedance. Thus, an increase in maximum output level can be achieved therewith.
As shown in FIG. 2, another embodiment of the invention disclosed in U.S. Pat. No. 6,453,048 includes: a first amplifier tube 2 which is the above-described amplifier tube in cathode follower connection; and a second amplifier tube 4 in cascade connection with the first amplifier tube 2. FIG. 2 shows a first diode 1A, a second diode 1B, a third diode 1E, a capacitor 1D, a capacitor microphone unit 100, an input terminal 4A, a ground side input terminal 4B, an input terminal 4C for a high voltage direct power supply, an output terminal 4D, and a ground terminal 4E. The second amplifier tube 4 is a triode. The cathode of the first amplifier tube 2 is connected to the plate of the second amplifier tube 4 via the third diode 1E in forward direction. A resistor 5 of a small resistance is connected between the high voltage power supply and the plate of the first amplifier tube 2. The plate of the first amplifier tube 2 is connected to the grid of the second amplifier tube 4 via a capacitor 6. A resistor 7 is connected between the ground and the cathode of the second amplifier tube 4. A circuit formed of the second amplifier tube 4 and the resistor 7 functions as a constant current load. The cathode of the second amplifier tube 4 is connected to the output terminal 40 so that an output signal is obtained from the cathode of the second amplifier tube 4. As described above, this embodiment of the impedance converter according to the invention disclosed in U.S. Pat. No. 6,453,048 aims to further lower the output impedance by cascade connection between the two amplifier tubes 2 and 4.
In the impedance converter according to the embodiment described in U.S. Pat. No. 6,453,048, the same amount of current, which is defined by the third diode 1E, flows in the first and the second amplifier tubes 2 and 4 due to the cascade connection. Unfortunately, the amplifier tubes 2 and 4 each formed of a vacuum tube have highly variable characteristics, and the potential of the output signal extracted from the output terminal 4D is difficult to be maintained at a constant level. Further, noise may be produced as a potential difference between the cathode and the heater of the first amplifier tube 2 becomes large to cause dielectric breakdown therebetween. A current flowing between the plate and the cathode of the first amplifier tube 2 is variable due to highly variable characteristics of the first and the second amplifier tubes 2 and 4 each formed of a triode. Accordingly, even if a constant bias voltage is applied to the amplifier tube 2 with the third diode 1E, the plate current and the output signal are variable.
Further, heater current needs to be supplied to both amplifier tubes 2 and 4. Thus, power consumption is high.